The present invention relates to impregnation or extraction of wood using a supercritical fluid as carrier for the substance impregnated into the wood or as extractive medium.
More particularly the invention relates to impregnating or extracting treatment of resin-containing wood and enables an expansion of the field of wood treatments using fluids in supercritical state.
Use of fluids under supercritical conditions offers substantial advantages in operations involving perfusing of a porous material for extractive purposes or for impregnation.
The advantages of using fluids under supercritical conditions over conventional processes using organic solvents or water as extracting or carrying medium at conditions of temperature and pressure in which the liquid stage is maintained, can be important and include the following features.
Supercritical fluids, possibly including minor amounts of co-solvents, are able to perfuse or penetrate porous materials quicker and more efficient than liquids, thereby enabling a more uniform impregnation or extraction in the interior of the material being treated and also enabling impregnation or extraction of materials regarded as a nearly impermeable to liquids.
The fact that supercritical fluids are almost as dispersible as gases facilitates an even contact with the porous substrate to be treated. Further, the fact that the solubility of several substances in supercritical fluids is highly pressure dependent enables an efficient deposition of such substances in the interior of the porous substances by pressure reduction following impregnation with supercritical solutions at higher pressures.
Supercritical fluids have also been suggested for the extraction, and especially the impregnation, of wood where the potential advantages include not only improved efficiency of the treatments but also involve substantial environmental improvements both in the performing of the treatment and possible post conditioning and in the subsequent use and disposal of the treated wood articles.
For further description of supercritical fluid treatments of wood materials reference is made to the following.
An article of Morrell and Levien: xe2x80x9cDevelopment of New Treatment Processes for Wood Protectionxe2x80x9d Conference Report from xe2x80x9cConference on Wood Preservation in the ""90s and Beyondxe2x80x9d, Savannah, Georgia USA, Sep. 26-28 1994 which deals with impregnation of wood species normally resistant to impregnation, by using supercritical carbon dioxide to deliver and deposit biocides into said wood. The potential for completely impregnating virtually all wood species also with biocides not previously regarded as suitable, is discussed. The supercritical fluid treatments are described as representing the first truly revolutionary improvement in treatment in this century, although it is admitted that a substantial amount of research and testing will be required before these systems become commercially feasible.
Also a paper by Hervxc3xa9 van Oost, Philippe Eymard and Michel Gastiger: xe2x80x9cTraitement de l"" xc3xa9picxc3xa9a en milieu supercritiquexe2x80x9d, Info Critt No. 6, 1995, provides a general description of the use of supercritical fluids for conservating treatment of wood, especially spruce. Based on laboratory experiments using carbon dioxide as supercritical fluid with possible addition of alcohol it is expected that the technique could be developed into commercial scale not only for introducing pesticides, but also for impregnation of wood with a view of improving physical characteristics thereof.
U.S. Pat. No. 5,094,892 comprises a review of prior art methods utilizing supercritical fluids for various purposes comprising deposition of various materials into a porous substrate or extraction of materials from such substrates. The latter process may be performed to recover valuable extracts or to improve characteristics of the substrate. The patent concentrates on the improvement obtainable by using co-solvents when perfusing wood, using typically carbon dioxide as supercritical fluid. Among the advantages also this patent emphasizes a uniformly impregnating of otherwise difficultly permeable materials.
Similar information can be found in U.S. Pat. No. 4,992,308 which i.a. describes impregnation using monomers which are polymerized in situ.
U.S. Pat. Nos 5,364,475 and 5,476,975 both deal with the extraction of organic toxic contaminations from wood using supercritical carbon dioxide.
Also delignification of wood has been suggested in the above mentioned U.S. Pat. No. 4,992,308 and in U.S. Pat. No. 5,041,192.
WO-A-95/34360 describes an analytical extraction process using a solvent under high temperature and pressure but not in the supercritical state. After the extraction a purging or flushing step is performed which transports the extraction fluid into a collection chamber. The purging or flushing step utilize another fluid than the one used in the extraction process.
U.S. Pat. No. 5,785,856 describes an apparatus that is particular adapted to perform the extraction process described in WO-A-95/34360 in an automated fashion, but which also may be operated under supercritical conditions. An example of suitable purging fluid is nitrogen under high pressure. It is noted that beside transporting the extraction fluid to a collection chamber the purging step has the further advantage of drying the extracted material.
U.S. Pat. No. 4,308,200 describes a process for extraction of coniferous woods with supercritical fluids to recover tall oil and turpentine or compounds thereof. It is described that the extract bearing fluid is stripped of the extracts by reducing the pressure in stages, each pressure reduction effecting removal of extracts of a defined molecular weight range.
In spite of the fact that treatment of various materials by perfusion thereof to perform extraction or impregnation, in principle may advantageously be carried out using a supercritical fluid as carrier in the perfusion process, such processes have hitherto not found commercial application, at least not at the level which could be expected in view of their potential advantages.
Especially within one of the largest potential application areas, namely in the treatment of wood substrates, these processes have, to the best knowledge of the present inventors, not achieved large-scale commercial use.
The present inventors have conducted extensive research with a view of developing and improving processes of the discussed type, expecially for treatment of resin-containing wood substrates.
In the present specification and the attached claims the term xe2x80x9cwood substratexe2x80x9d designates a substrate for the impregnation or extractive process which may typically be a shaped or partially shaped wood article, structural wood, timber, poles etcetera, but encompasses also materials comprising comminuted wood such as chips or building plates etcetera.
By said research and experiments it has turned out that an important feature which may be at least partly responsible for the lacking or very restricted commercial application of perfusion processes using supercritical fluids in wood products, is the contents of resin in most of such wood products. Such resin may under the influence of the supercritical fluid cause deterioration of the resulting products and/or operational complications.
In this context the term xe2x80x9cresinxe2x80x9d denotes the high viscous liquid of lipophilic or hydrophobic character present in amounts of typically some percent by weight in most types of wood, especially in wood from coniferous tres. Such resin is a very complex mixture of various substances including relative volatile components such as terpenes, whereas the main component is a mixture of non-volatile, partly unsaturated compounds including esters and free acids. The resin forms an extremely sticky gum which is capable of undergoing a certain slow hardening when exposed to the air.
The resin is normally present as small drops within the cells forming the wood structure.
Most of the substances coming into consideration as supercritical fluid in the wood perfusing processes, coming into consideration herein, including primarily carbon dioxide and hydrocarbons, such as ethane, propane and buthylene, as well as certain auxiliary substances suitable as adjuvants in the fluid, are soluble in resin and during the extractive or impregnative perfusion processes a substantial amount thereof is dissolved in the resin present in the interior of the wood.
As can be shown in experiments using samples of pure resin extracted from wood, the viscosity and surface tension of the resin are such that carbon dioxide or volatile hydrocarbons dissolved therein at high pressure in the supercritical perfusion process only escapes slowly when the pressure is reduced and therefore the pressure reduction involves extensive formation of bubbles and foam.
When the superatmospheric pressure, typically 50-100 bar, used in the hitherto suggested processes for perfusion of wood substrates, is released, a similar phenomenon occurs and the bubble formation in the individual droplets of resin causes the resin to be exudated to the surface of the wood substrate from where a part of it may be entrained by the leaving fluid and form deposits on the interior walls and exhaust pipes of the treatment chamber.
In case the wood substrate is a shaped wood article, the resin present as a layer on the surface thereof after termination of the treatment prevents immediate application of further finishing treatments, such as painting, varnishing etcetera, and the surface achieves an inattractive sticky character.
Also on timber and constructional wood intended for subsequent shaping operations the presence of the resin on the surface will often be unacceptable.
Due to the physical character and insolubility in water of the resin, deposits thereof in the chamber and connected pipes may create substantial operational problems and expenditure. These last mentioned problems also exist when the wood substrate is comminuted wood, such as chips or building plates comprising comminuted wood. As mentioned above such materials may be treated for extractive or impregnative purposes.
The higher the maximum pressure is in the perfusion process the more pronounced and disturbing are the problems caused by resin exudation. Thus, said problems have in fact made the process unattractive for treating certain difficultly perfusable substrates requiring very high pressure for effective treatment.
As a first attempt to avoid or reduce the problems caused by the above exudation of the resin from the interior of the wood substrate experiments have been made using very slow and thus prolonged exhaustion of the fluid in supercritical state and thus a very slow pressure release.
Although this measure in principle is efficient to mitigate the problems caused by exorbation of resin, it has turned out that to reduce said exorbation sufficiently the pressure release has to be so slow that the time period necessary for completing said pressure release before emptying the treatment chamber becomes so extended that the total capacity of the process and the plant used therein is decreased to levels seriously impairing the competitiveness of the total extraction or impregnation process.
Thus, there is a substantial need for measures to avoid excessive exorbation of resin from wood substrates when these are subjected to pressure release after supercritical fluid perfusion operations, without the necessity of using a prolonged release time. Avoidance of resin exorbation would not only solve or diminish the above problems but also widen the area of applicability for the processes to substrates which can only be perfused at very high pressures, such as wood having a high proportion of heartwood.
One approach to fulfil this need and meet the described problems is subject of the co-pending Danish patent application No. 1455/98, filed the same date as the present application. The present invention fulfils said need using different measures.
The present invention is based on the recognition that during the pressure release the portion of the fluid used in supercritical state, which is dissolved in the resin, may be allowed to evaporate therefrom relatively fast without formation of bubbles and resin exorbation, if the partial pressure of the substance forming the supercritical fluid is reduced stepwise with intermediate pressure increase as defined below.
Thus, the present invention deals with a method of performing an impregnating or extracting treatment on a resin-containing wood substrate using a fluid in supercritical state as delivering or extractive solvent medium which fluid in supercritical state is soluble in the resin present in the wood substrate, comprising the steps of
(i) introducing the wood substrate into a pressure tight treatment chamber,
(ii) introducing a stream comprising said fluid into the chamber and adjusting the pressure and temperature therein to ensure the fluid being present in supercritical state and to promote penetration of the fluid and any substances dissolved therein into the wood substrate,
(iii) maintaining contact between the wood substrate and fluid in supercritical state for a time period sufficient to obtain the desired penetration, whereby also a certain dissolution of the fluid into the resin takes place,
(iv) after a possible purging of the chamber with said fluid in supercritical state, releasing the pressure in the chamber down to ambient pressure, and
(v) withdrawing the treated wood substrate from the chamber,
and the method is characterized in that the releasing step (iv) comprises the features:
(a) starting releasing the pressure at a rate which, if it were continued until atmospheric pressure were reached, would cause exudation of resin from the interior of the wood substrate to the surface thereof due to bubble forming expansion of the fluid dissolved in the resin in step (iii),
(b) interrupting said releasing at a first pressure level before harmful exudation of resin to the surface of the wood substrate takes place,
(c) increasing the pressure to a second level above said first level but below the pressure maintained in step (iii),
(d) immediately after reaching said second pressure level resuming pressure release to decrease the pressure to a third level below said first level,
(e) subsequently at least once repeating a release operation comprising pressure increase to a level below the maximum level of the immediately preceding release operation, instantly followed by pressure decrease to a level below the minimum level of the immediately preceding release operation,
wherein the number of release operations in (e) and the pressure levels in each of these operations as well as in the steps (b), (c) and (d) are previously fixed on basis of simple experiments, using relevant specimens of wood or resin, to obtain a shorter total release time (a)+, (b)+, (c)+, (d)+ and (e)+ without harmful resin exudation, than would have been possible if the pressure were released by a continous, uninterrupted withdrawal of the fluid.
In one of the preferred embodiments of the method the increase of pressure in (c) and preferably also in (e) is performed by introducing the same species of fluid as used as delivering or extracting supercritical solvent medium. Thereby it is omitted that the gas flow recovered from the method is mixed up with foreign gases. Therefore it can be reused without extensive separation or purification, and the method can be performed with only a very moderate waste of the gas used as supercritical solvent medium.
However, in case a very short process time and thus a fast pressure decrease is paramount, the process of the invention can be embodied using the principle forming basis for the above mentioned co-pending Danish patent application incorporated herein by reference. In this embodiment the increase of pressure in (c) and also in (e) is performed by introducing a fluid which is less soluble in resin than the fluid used as delivering or extracting solvent medium. Thereby the number of release operations and pressure increase operations specified above under (e) can be reduced and/or the pace of each pressure releasing operation can be increased to obtain a total reduced process time.
As it appears from the introductory portion of this specification, the method of this invention involves advantages for both impregnation and extraction processes in connection with substrates comprising articles of wood as well as comminuted wood materials and articles comprising such. However, currently most experiments and experiences have been obtained in connection with impregnation of wood as such, and thus a preferred embodiment of the process is characterized in that a resinous wood is impregnated with one or more biocides such as fungicides or insecticides. Tests have especially been carried out using wood from a coniferous tree, preferably selected from spruce (pica), fir (abies, pseudotsuga), hemlock (tsuga) and pine (pinus) including larch (larix), which is impregnated using a wood preserving agent comprising at least one fungicide or other biocide.
The process may also be advantageous for treating hardwood, such as beechwood, to obtain a uniform dying through the complete interior thereof.
Due to physical and chemical properties as well as availability and costs and lacking toxicity and non-flammability, carbon dioxide, possibly together with a minor amount of a solubility promotor such as an alcohol or ketone, is the preferred fluid used in supercritical state when the purpose is to impregnate wood by means of an organic fungicide or insecticide. However, hydrocarbons can also be used for this purpose, especially such having from 2-4 carbon atoms.
However, such hydrocarbons are easily soluble in resin just as carbon dioxide is, and their release from said resin, when the pressure is reduced, may cause the problems explained above.
If the increase of pressure in (c) and (d) is performed by introducing a fluid which is less soluble in resin than the fluid used as delivering and extracting medium, this fluid may typically be nitrogen or atmospheric air which do not dissolve in the resin to such an extent that their release therefrom causes problems.
In a typical application of the process resinous wood from a coniferous tree is impregnated with at least one organic biocide using carbon dioxide as the supercritical fluid acting as delivering solvent medium, and the contact in step (iii) is maintained for 5-60, preferably 10-30, minutes at a pressure of 20-500, preferably 50-400, more preferably 60-150 bar and at a temperature of 31-80xc2x0 C., preferably 31-65xc2x0 C., and the step (iv) comprising the features (a), (b), (c) and (d) is completed within a period of 0.5-5 h, preferably 1.5-4 h, more preferably 100-200 minutes.
Typically the feature (e) comprises 1-20, preferably 3-10, operations, each consisting of a relatively fast pressure increasing measure and a slower pressure decreasing measure.
Addition of certain organic solvents to the supercritical fluid, especially when the latter is carbon dioxide, has been described as widening the pores of wood substrates to be perfused. Further, such solvents may be selected to improve the solubility of certain biocides or other substances which it is desired to impregnate into the wood substrate.
Thus, a preferred embodiment of the method is characterized in that to increase the delivering or extractive ability of the fluid in supercritical state an organic co-solvent is added to said fluid.
Solvents can also be used with the purpose of bringing the substance(s) to be infused into the substrate in liquid, low viscous state to facilitate handling and especially dosing thereof.
In case the method is used for impregnating wood substrates to resist attack from fungi and/or insects, several biocides come into consideration.
Thus, as example of suitable fungicides copper salts, such as copper naphtenate and copper linolate and similar derivatives may be mentioned.
Also propiconazole or tebuconazole are fungicides which currently are accepted and commercially used for wood impregnation.
Experiments have shown that these two fungicides by the method of the invention using carbon dioxide as supercritical fluid can be dispersed evenly in the wood in concentrations sufficient for the desired preservation. Especially a combination of propiconazole and tebuconazole seems suitable.
However, the process of the invention is in no way restricted to biocide impregnation of wood substrate, but it is also suitable for impregnation of wood substrate with one or more of the species of the groups: colorants, fireproofing agents, and other agents imparting specific qualities, e.g. strength-improving agents such as agents which are polymerized in situ after having been dispersed within the wood structure.
A further example of a field of application for the method is extraction of a wood substrate to remove components therein, which would cause discoloration such as certain metal compounds and tannin-like compounds. Also extraction of valuable wood components comes into consideration.